The present invention relates to a centrifugal fan which includes vanes, for example, forward curved vanes, radial curved vanes or backward curved vanes and, in particular, relates to a centrifugal fan with a noise suppressing arrangement suitable for house-hold use and for cooling business machines, computers and, particularly, air conditioners in motor vehicles.
A conventional centrifugal fan, such as disclosed in Japanese Utility Model Laid-Open No. 48-63305 (1973), is constructed by a fan, a scroll casing accommodating the fan and a motor for driving the fan. In such a construction, the fan suctions air from the axial direction and discharges the air in the radial direction of the fan. Namely, the centrifugal fan is constructed to discharge the air in a centrifugal direction. This scroll casing gradually restores the dynamic pressure caused by rotating the fan to a static pressure within the scroll casing.
However, in such structure, no counter-measure is taken for reducing an air pulsating sound caused by the fan, a vortex sound caused by wind separation from the vanes and a turbulent sound within the scroll casing.
Japanese Utility Model Publication No. 57-24566 (1982) discloses an air conditioner for use in an automobile wherein an inner and outer air changeover box is modified so as to indirectly reduce such noises.
Patent Application Laid-Open No. 63-90415 (1988) discloses an air conditioner for use in an automobile, wherein a sound absorbing member which is formed by adhering a thin elastic nonporous member on a surface of a thick elastic porous member is attached to the bottom wall of the scroll casing of the centrifugal fan for reducing noise.
In the conventional centrifugal fan disclosed above, Japanese Utility Model Laid-Open No. 48-63305 (1973), the air pulsating sound caused by the fan, the vortex sound caused by wind separation from vanes and the turbulent sound within the scroll casing are not specifically suppressed, resulting in a total noise level from the centrifugal fan which is comparatively high.
With regard to the air conditioner for use in an automobile as disclosed in Japanese Utility Model Publication No. 57-24566 (1982), the modification of the inner and outer air changeover box results in a complex structure, and the production cost increases; and the mounting condition thereof lowers the versatility.
Further, in the scroll casing of the centrifugal fan disclosed in Japanese Patent Application Laid-Open No. 63-90415 (1988), the sound absorbing member adheres to the bottom of the scroll casing to reduce the noise. However, a soft polyurethane foam which is generally used as a sound absorbing material particularly absorbs sounds having high frequency components of more than 1000 Hz and generally does not absorb sound having low frequency components, for example, less than 1000 Hz. In the Japanese Patent Application Laid-Open No. 63-90415 (1988), the elimination of the above problems of the sound absorbing member is disclosed by adhering a thin plate of an elastic nonporous material such as chloroprene rubber plate on one of the surfaces of a thick plate of an elastic porous material such as soft polyurethane foam. With the use of such sound absorbing member, the vibrating energy of the sound waves covering a broad range of the frequency components can be absorbed by the two kinds of sound absorbing materials adhered having different sound absorbing characteristics. With such a sound absorbing member, it is possible to absorb the vibrating energy of low frequency sound waves because of the use of the elastic non-porous material on the surface of the combined sound absorbing member; however, the unabsorbed sound waves can not reach the lower elastic porous material; quite the contrary, the unabsorbed sound waves are reflected at the interface such that the substantial benefit of the elastic porous material disposed in the lower side is lost compared with the effect obtainable by a mere provision of the thick elastic porous material. Therefore, the sound waves of the high frequency components can not be reduced.
Further, for maximizing the effect of the sound absorbing member, it is necessary to provide the sound absorbing member over the entire inner wall surface of the scroll casing such that the thin plate is difficult to adhere to curved surfaces which increases the production cost; further there is a problem that the sound absorbing member such as the thin plate, which is always subject to the wind flow, may peel off if no countermeasure are taken.